Dushman is copied from hollywood movie Eye for an Eye. Sonia and Naina Saigal are look alike twins, but that's where the resemblance ends, for by nature both are very different. Naina is more traditional by nature, but Sonia is more outgoing, and precocious.
Kabir is in love with Sonia, but is unable to express his feelings for her, and takes the assistance of Naina to resolve this. At Naina's instructions, Kabir is able to propose to Sonia, and to his delight, Sonia's accepts. Then everything changes for the Saigal family, as Sonia is brutally murdered, and the killer's next target is Naina, who must now re-locate and hide from this elusive killer, who is determined to hunt her down ruthlessly.